INTRODUCTION

This paper reports new paleomagnetic, palynologic, radiometric, and geochemical data related to the Paleocene age of the dinosaur-bearing Ojo Alamo Sandstone and Animas Formation in the San Juan Basin of New Mexico and Colorado. These data provide the primary evidence for the ages of rock strata adjacent to the K-T interface in the San Juan Basin.

Because the Ojo Alamo Sandstone contains in situ dinosaur fossils, its Paleocene age has been questioned over the years. Multiple workers, beginning with Reeside (1924), suggested (or implied) that the dinosaur fossils of the Animas Formation and the Ojo Alamo Sandstone were Paleocene in age, however until recently, the evidence for the Paleocene age of the Ojo Alamo has been suggestive, but not entirely conclusive (Fassett 1982, 1987). Fassett and Lucas (2000) and Fassett et al. (2002), however, presented new data supporting the Paleocene age of the Ojo Alamo Sandstone. Fassett et al. (2002) presented geochemical data showing that all dinosaur fossils from the Ojo Alamo Sandstone that were analyzed, had been mineralized in place during Paleocene time and thus could not have been reworked from underlying Cretaceous strata. These new data, plus the expanded paleontologic, paleomagnetic, and geochemical analyses presented in this report, fully support earlier conclusions of Fassett and Lucas (2000) and Fassett et al. (2002) that some dinosaurs lived on into earliest Paleocene time in the San Juan Basin area. This study shows that these Lazarus dinosaurs lived for as long as 0.5 m.y. into Paleocene time. The presence of dinosaur fossils in the Paleocene Animas Formation of the northern San Juan Basin, first noted by Reeside (1924), seems to have been forgotten or ignored since that time; a discussion of Reeside's data plus new information related to Animas Formation dinosaur fossils are presented herein.